1. Field of the Invention
The present invention relates to a method for preparing a substrate having a monocrystalline film with few crystal defects.
2. Description of the Related Art
Importance for monocrystalline films and substrates formed of silicon, SiC, GaN, AlN, zinc oxide, diamond or the like used in semiconductor devices such as power transistors, lasers, LEDs and high-frequency elements, has been more and more increasing.
Typically, a monocrystalline film is prepared by growing a film by vapor phase epitaxial, liquid phase epitaxial, sputtering, EB, MBE, PVD such as sublimation or the like on a monocrystalline substrate selected from for example, silicon, sapphire, SiC or the like having a lattice constant close to that of the monocrystalline film.
On the other hand, the substrates used in these examples are generally prepared by bulk crystal growth in an FZ method, a CZ method, a sublimation method or the like using a seed crystal, followed by slicing, polishing or the like.
However, the monocrystalline film or the substrate obtained by such conventional methods has disadvantages in that dislocation defects in the monocrystalline substrate used as a seed substrate are succeeded, or crystal strain or a lamination defect caused by mismatching of the lattice constant or the coefficient of thermal expansion, or crystal defect such as micro-pipe is apt to occur easily. The dislocation defects mean the disorder of periodicity of atoms in the crystal.
Since a large number of these crystal defects have an adverse effect on the initial properties and the long-term reliability of a device, the crystal defects of the monocrystalline film or the substrate to be used have to be as few as possible in order to prepare a semiconductor device having high performance and high reliability.
Therefore, conventionally, in order to decrease the number of crystal defects, an extremely expensive monocrystalline silicon having nearly no crystal defects (near perfect crystal) has been used. Alternatively, a number of buffer layers are formed between a monocrystalline substrate and a monocrystalline film to be grown. The buffer layers such as layers of SiO2, silicon, GaN, AlGaN, InGaN or GaAs, which have middle values of the lattice constant and the coefficient of thermal expansion between those of the monocrystalline substrate and the monocrystalline film to be grown, are formed on the monocrystalline substrate and followed by growth of the monocrystalline film on the surface of the buffer layers (see JP 2004-048076A).
However, these improvements require high material costs and process costs and are economically disadvantageous. There is concern over safety and health matters and the reduction of crystal defects is insufficient. Accordingly, they are not practical.